Conductive soil-repellent core-sheath fiber of high chemical resistance, its preparation and use

ABSTRACT

Conductive soil-repellent core-sheath fiber of high chemical resistance, its preparation and use  
     Described is a melt-spun fiber having a core-sheath structure and a high tensile strength whose core contains a synthetic thermoplastic polymer which is not a fluoropolymer and whose sheath contains at least one melt-spinnable fluoropolymer and particles comprising electroconductive material.  
     The fibers of the invention are useful especially in the form of monofilaments for producing textile fabrics for industrial applications

[0001] The present invention relates to conductive soil-repellentcore-sheath fibers, especially monofilaments, which are useful inindustrial fabrics in particular.

[0002] It is known that fluoropolymers have good thermal stability, goodchemical resistance and soil-repellent properties. It has already beenattempted to process melt-spinnable fluoropolymers into fibers, multi-and monofilaments in order that textile fabrics for industrialapplications that have the abovementioned properties of fluoropolymersmay be manufactured therefrom. The disadvantage with melt-spinnablefluoropolymers is the high creep. Fibers and filaments formed from thismaterial therefore have only low tensile strengths and are not shapestable.

[0003] It has also already been tried to combine fluoropolymers withpolymers having good mechanical performance characteristics, for examplewith polyethylene terephthalate (hereinafter also referred to as PET).However, it is to be noted in this context that fluoropolymers are oftenincompatible with other polymers in that they generally do not mix withthem. The result is thus frequently a two-phase mixture in which thefluoropolymers form three-dimensional islands. The weight fraction offluoropolymer which can be added is thus frequently limited, since theboundary layers of the polymers have only poor adhesion to each other.This property manifests itself in fibers as a tendency to split.

[0004] Monofilaments composed of PET and random copolymers of ethyleneand tetrafluoroethylene (ETFE) have been commercially available foryears. Frequently, these fibers have a low carboxyl end group content tostabilize them against hydrolysis and are stabilized with carbodiimidesto cap the carboxyl end groups. The capping of carboxyl groups by meansof carbodiimides is described for example in EP-A417,717 andEP-A-503,421.

[0005] Industrial fabrics woven from these monofilaments do largely havethe mechanical properties of a PET filament, but combined with increasedhydrolysis resistance and improved soil repellency.

[0006] Because the fluoropolymer fraction is relatively low, the thermalstability and chemical resistance of these fibers are substantiallyequal to the data for pure PET. However, the increased tendency to splitcan manifest itself under mechanical stresses, for example at thebeating up of the fell on the weaving machine.

[0007] Fibers composed of synthetic polymers and woven fabrics producedtherefrom have the disadvantage, however, that they can become chargedup with static electricity as a result of friction. Conductive fibersfor producing textile fabrics, such as wovens for industrial use, or forother applications, for example brushes, have long been the goal ofnumerous developments.

[0008] DE-A-1 98 26 120 discloses a flame-retardant electroconductivewoven fabric which contains electroconductive and flameproofednonelectroconductive fibers. The electroconductive fibers can containelectroconductive particles, such as carbon black or metal particles, becoated with metal or consist of electroconductive materials, such aspolyanilines. The fiber materials mentioned are polyester andpolyolefins.

[0009] DE-U-86 238 79 discloses yarn for the manufacture of spiral tapeswhich is sheathed with a layer of curable polymer. This layer containselectroconductive carbon. Melamine resins, epoxides, phenolic resins orpolyurethanes are mentioned by way of example as polymers for the sheathlayer.

[0010] DE-A-39 38 414 describes high-strength woven fabric which isformed from artificial fibers, these being incorporated in the form ofelectroconductive fibers in the warp and the weft, and which containsnonelectroconductive fibers as well. The electroconductive fibersconsist of polyolefins and contain graphite or carbon black.

[0011] EP-A-160,320 describes hairbrush core-sheath monofilamentscomposed of selected polymers. The core contains nylon or polyesterswhich comprise at least 60% by weight of polybutylene terephthalateunits. The sheath contains specific nylon grades or copolyetherester.

[0012] DE-U-86/06334 discloses core-sheath fibers whose core consists ofthermoplastic polymer, preferably of polyamide, and whose sheathconsists of electroconductive polymer, preferably of polyamide, whichcontains embedded carbon black or metals.

[0013] JP-A-07/278,956 describes electroconductive copolyesters whichmainly contain polybutylene terephthalate units and which are doped withcarbon black. It also describes core-sheath fibers composed of thismaterial which have a core which consists of aromatic polyester.

[0014] WO-A-98/14,647 discloses electroconductive heterofilaments whichcan be configured as core-sheath fibers for example. Examples describedof core and sheath polymers are PET and other polyesters or PET/nylon.

[0015] WO-A-01/20,076 discloses nonwovens having a high dielectricconstant. Mixtures of polyvinylidene fluoride and polypropylene areproposed as fiber material. The products formed therefrom are notablefor a long half-life for electrostatic charges and can be used aselectrostatic filters.

[0016] U.S. Pat. No. 6,085,061 describes a brush for cleaningelectrostatically charged surfaces. The fiber materials used can becore-sheath fibers whose core is electroconductive and whose sheathconsists of polyvinylidene fluoride.

[0017] DE-A44 12 396 discloses melt-spun undrawn nonelectroconductivefibers having a core-sheath structure whose sheath containsfluoropolymers. The core polymer used is polycarbonate. This fiber isused as an optical fiber and is unsuitable for industrial textiles, suchas industrial wovens for example, on account of its low strengths.Moreover, the critical properties with an optical fiber are highreflection at the boundary layer and very low attenuation of theelectromagnetic wave. Both these properties can only be achieved throughuse of a high-purity coating.

[0018] JP-A-2001-127,218 describes a semiconducting fiber composed of afluoropolymer which contains carbon black. This fiber does not have acore-sheath structure and is used for manufacturing nonwovens, forexample by the melt-blow process. The fiber has not been drawn.

[0019] It is an object of the present invention to combine theperformance advantages of the materials known for fiber productionwithout having to incur the disadvantages associated with the use of theindividual materials.

[0020] A person skilled in the art knows that bonding problems arenormal at the boundary layer between two polymers. This holds especiallyfor the use of known poor-bonding fluoropolymers with other polymers. Ithas been determined that, surprisingly, the use of a fluoropolymer whichis doped with electroconductive particles provides very good bonding tothe polymer core.

[0021] It is thus a further object of the present invention to providecore-sheath fibers which possess good bonding between the individuallayers.

[0022] The present invention thus provides fibers, especiallymonofilaments, which combine antistatic properties with high chemicaland thermal stability and good mechanical shape stability and also hightensile strength.

[0023] This invention is a melt-spun fiber having a core-sheathstructure and a tensile strength of at least 15 cN/tex whose corecontains a synthetic thermoplastic polymer which is not a fluoropolymerand whose sheath contains at least one melt-spinnable fluoropolymer andparticles comprising electroconductive material.

[0024] The synthetic thermoplastic polymers forming the core are freelychooseable, as long as they are melt spinnable and provide the fiberwith the properties desired for the particular intended application.Fluoropolymers are not comprehended by synthetic thermoplastic polymers,even though the core may contain fluoropolymers as a blend component aswell as synthetic thermoplastic polymers.

[0025] Examples of synthetic thermoplastic materials are polyolefins,such as polyethylene, polypropylene or copolymers containing ethyleneand/or propylene units combined with other copolymerized alpha-olefinunits, such as alpha-butylene, alpha-pentylene, alpha-hexylene oralpha-octylene; polyesters, such as polycarbonate, aliphaticallyaromatic polyesters or wholly aromatic polyesters; polyamides, such asaliphatic or aliphatically aromatic polyamides (nylons) or whollyaromatic polyamides (aramids); or polyether ketones, ie polymers whichhave at least ether and ketone groups and generally aromatic divalentradicals, such as phenylene, in the recurring chain, many combinationsof these groups being possible, for example PEK, PEEK or PEKK; orpolyarylene sulfides, preferably polyphenylene sulfide; or polyetheresters, ie polymers which have at least ether and ester groups andgenerally aromatic divalent radicals, such as phenylene, in therecurring chain, for example TPE-E; or polyacrylonitrile orpolyacrylonitrile copolymers with other ethylenically unsaturatedcomonomers, such as acrylic or methacrylic acid.

[0026] Preferably, the core of the core-sheath fibers of this inventioncontains polyamides and especially polyesters.

[0027] The thermoplastic polyamides which are preferably used in thecompositions of the present invention are known per se.

[0028] Examples thereof are fiber-forming polyamides, such as aliphaticor aliphatically aromatic polyamides, for example polycaprolactam,poly(hexamethylene-1,6-diamineadipamide),poly(hexamethylene-1,6-diaminesebacamide),poly(hexamethylene-1,6-diamineterephthalamide) orpoly(hexamethylene-1,6-diamineisophthalamide); or else wholly aromaticpolyamides, such as poly(phenylene-1,4-diamineterephthalamide) orpoly(phenylene-1,4-diamineisophthalamide).

[0029] The polyamides used in this invention have DIN 53727 viscositynumbers which are customarily in the range from 120 to 350 andpreferably from 150 to 320 cm³/g (measured at 25° C. in sulfuric acid).

[0030] The thermoplastic polyesters and/or aromatic liquid-crystallinepolyesters which are more preferably used in the compositions of thepresent invention are known per se.

[0031] Examples thereof are fiber-forming polyesters, such aspolycarbonate or aliphatically aromatic polyesters, for examplepolybutylene terephthalate, polycyclohexanedimethyl terephthalate,polyethylene naphthalate or especially polyethylene terephthalate, orelse wholly aromatic, liquid-crystalline polyesters, such aspolyoxybenzonaphthoate. Building blocks of fiber-forming polyesters arepreferably diols and dicarboxylic acids or appropriately constructedhydroxy carboxylic acids. The main acid constituent of the polyesters isterephthalic acid or cyclohexanedicarboxylic acid, but other aromaticand/or aliphatic or cycloaliphatic dicarboxylic acids may be suitable aswell, preferably para- or trans-disposed aromatic compounds, such as forexample 2,6-naphthalenedicarboxylic acid or 4,4′-biphenyldicarboxylicacid, or else p-hydroxybenzoic acid. Aliphatic dicarboxylic acids, suchas adipic acid or sebacic acid for example, are preferably used incombination with aromatic dicarboxylic acids.

[0032] Typical suitable dihydric alcohols are aliphatic and/orcycloaliphatic and/or aromatic diols, for example ethylene glycol,propanediol, 1,4-butanediol, 1,4-cyclohexanedimethanol or elsehydroquinone. Preference is given to aliphatic diols which have 2 to 4carbon atoms, especially ethylene glycol; preference is further given tocycloaliphatic diols, such as 1,4-cyclohexanedimethanol.

[0033] Preferred thermoplastic polyesters are especially selected fromthe group consisting of polyethylene terephthalate, polyethylenenaphthalate, polybutylene naphthalate, polypropylene terephthalate,polybutylene terephthalate, polycyclohexanedimethanol terephthalate,polycarbonate or a copolycondensate containing polybutylene glycol,terephthalic acid and naphthalenedicarboxylic acid units.

[0034] Further preferred thermoplastic polyesters are aromatic,liquid-crystalline polyesters, especially polyesters containingp-hydroxybenzoate units.

[0035] Especially in the case of fibers which are to be used in hotmoist environments, such as monofilaments for use in paper machines, andwhich contain polyesters as a core component, these polyesters arepreferably stabilized against hydrolytic degradation by addition ofpolyester stabilizers.

[0036] Such stabilized fibers exhibit a significant reduction in thedegradation tendency of the polyester, so that monofilament lifetimescan be achieved which are equivalent to those of monofilaments based onextremely stable fiber materials, such as polyarylene sulfides oroxides.

[0037] Particular preference is given to fibers containing stabilizedpolyesters and particularly preferably carbodiimides in the core.

[0038] The polyesters used in the present invention typically havesolution viscosities (IV values) of at least 0.60 dl/g, preferably of0.60 to 1.05 dl/g, more preferably of 0.62-0.93 dl/g, (measured at 25°C. in dichloroacetic acid).

[0039] The fluoropolymers forming the sheath are likewise freelychooseable, as long as they are melt spinnable.

[0040] The fluoropolymers used in the present invention arepoly(fluoroolefin) homopolymers and/or copolymers derived fromethylenically unsaturated fluorous olefin monomers and other monomerswhich are copolymerizable therewith. Such polymers are likewise knownper se.

[0041] Examples thereof are melt-spinnable copolymers oftetrafluoroethylene with other alpha-olefins, such as ethylene,propylene, butylene, hexylene or octylene.

[0042] But it is also possible to use homo- or copolymers which arederived from other fluorous monomers, for example from mono-, di-,trifluoroethylene, from vinyl fluoride or especially from vinylidenefluoride.

[0043] Particular preference is given to using melt-spinnable copolymersof tetrafluoroethylene with at least one alpha-olefin, preferably withethylene.

[0044] Very particular preference is given to using polyvinylidenefluoride (PVDF) as a sheath component.

[0045] When spinning polyesters, especially PET, with PVDF to form abicomponent monofilament in a core-sheath structure there is a surprisein that very good core-sheath bonding is obtained.

[0046] The invention therefore also includes a heterofilament fibercontaining at least two components, the first component being anelectric insulator and comprising a thermoplastic polymer which is not afluoropolymer and the second component comprising polyvinylidenefluoride.

[0047] The particles composed of electroconductive material which arepresent in the sheath of the melt-spun fiber of the present inventionare freely chooseable, as long as they provide the sheath with anincreased electroconductivity.

[0048] The particles can be composed of carbon, being for example carbonfibers, carbon black or graphite; of metals, for example of copper,silver, aluminum or iron; of metal alloys, for example bronze; or ofconductive plastics, for example of polyanilines or of polypyrrole.

[0049] The particles can be present in any desired form, for example infiber form or in the form of round or irregular particles.

[0050] The level of electroconductive particles in the sheath is to bechosen such that a distinct increase in the electroconductivity of thepolymeric material results. Typical amounts vary in the range of up to50% by weight and preferably 2 to 15% by weight, based on the amount ofthe sheath material.

[0051] Particular preference is given to melt-spun fibers wherein thesheath contains between 2% by weight and 15% by weight and especiallybetween 4% by weight and 9% by weight of electroconductive particles.

[0052] The core-sheath fibers of the present invention can be present inany desired form, for example as multifilaments, as staple fibers orespecially as monofilaments.

[0053] The linear density of the core-sheath fibers of the presentinvention can likewise vary within wide limits. Examples thereof are 100to 45,000 dtex and especially 400 to 7,000 dtex.

[0054] Particular preference is given to monofilaments.

[0055] Particular preference is given to monofilaments whosecross-sectional shape is round, oval or n-gonal, where n is not lessthan 3.

[0056] The staple lengths in the case of staple fibers can likewise varywithin wide limits, for example between 30 to 70 mm.

[0057] The core of the core-sheath fiber of the present invention formsthe mechanical, load-bearing component of the fiber, whereas the sheathdetermines mainly the performance characteristics, such as antistaticperformance and lubricity. The core can preferably utilize acommercially available PET raw material.

[0058] The sheath more preferably utilizes a fluoropolymer based on PVDFwhich was previously processed with carbon black in particular into aspinnable mixture.

[0059] The weight fraction of the core-forming component A) to thesheath-forming component B) is, based on the total amount of thesecomponents, 50 to 95% by weight and preferably 60 to 80% by weight forcomponent A) and 50 to 5% by weight and preferably 40 to 20% by weightfor component B).

[0060] The core-sheath fibers of the present invention can be preparedaccording to processes known per se.

[0061] These processes comprise the measures of:

[0062] i) selecting a first polymer which is a synthetic thermoplasticpolymer and not a fluoropolymer and which has a first melting point,

[0063] ii) selecting a second polymer which contains particlescomprising electroconductive material and which is a melt-spinnablefluoropolymer which has a second melting point at least 20° C. below thefirst melting point,

[0064] iii) coextruding the first polymer and the second polymer througha heterofilament spinneret at a spinning temperature above the firstmelting point to form a bicomponent fiber having a core comprising thefirst polymer and a sheath comprising the second polymer and

[0065] iv) drawing the produced core-sheath filament to increase thetensile strength.

[0066] The two polymers or mixtures containing these polymers arepreferably dried directly before being fed into the extruder, melted inthe extruder and filtered through a spin pack. The fluoropolymer isprovided with the electroconductive particles. This is typicallyaccomplished before the fluoropolymer is fed to the extruder, but canalso take place directly upstream of the spin pack. It is similarlypossible to use masterbatches containing the fluoropolymer andelectroconductive particles.

[0067] After the polymer melts have been pressed through aheterofilament spinneret, the molten polymer thread is quenched in aspin bath, for example a water bath, and subsequently wound up or takenoff. The takeoff speed is greater than the ejection speed of the polymermelt and thus causes stretching or drawing of the extruded thread.

[0068] The as-spun heterofilament thread thus produced is subsequentlypreferably subjected to an afterdrawing operation, more preferably inplural stages, especially to a two- or three-stage afterdrawingoperation, to an overall draw ratio of 1:3 to 1:8 and preferably 1:4 to1:6.

[0069] Drawing is preferably followed by heat setting, for whichtemperatures of 130 to 280° C. are employed; the length is maintainedconstant or shrinkage of up to 30% is allowed.

[0070] It has been determined to be particularly advantageous for themonofilaments of the present invention to operate at a melt temperaturein the range from 285 to 315° C. and at a jet stretch ratio of 1:2 to1:6.

[0071] The spinning takeoff speed is customarily 10-40 m per minute.

[0072] When the thermoplastic polymer of the core and the fluoropolymerof the sheath are spun into a bicomponent monofilament in core-sheathstructure there is a surprise in that very good core-sheath bonding isobtained.

[0073] The conductivity of the sheath can be lost at drawing, but can berestored by a heat treatment and the thereby induced shrinkage,preferably above the melting point of the sheath material but below themelting temperature of the core.

[0074] The conductively doped fluoropolymer determines mainly thesurface properties. The fibers of the present invention are notable forvery good soil repellency, good chemical resistance andelectroconductivity.

[0075] The combination with the fluoropolymer leads to fibers havingimproved lubricity properties compared with fibers composed of straightthermoplastic polymer. These fibers exhibit enhanced soil repellencycompared with fibers composed of straight thermoplastic polymer.

[0076] The fibers of the present invention can contain assistants aswell as components A) and B). Examples of assistants are processingassistants, stabilizers, antioxidants, plasticizers, lubricants,pigments, delusterants, viscosity modifiers or crystallizationaccelerants.

[0077] Examples of processing assistants are siloxanes, waxes orlong-chain carboxylic acids or their salts, aliphatic, aromatic estersor ethers.

[0078] Examples of stabilizers and antioxidants are the abovementionedpolyester stabilizers, phosphorus compounds, such as phosphoric estersor carbodiimides.

[0079] Examples of pigments or delusterants are organic dye pigments ortitanium dioxide.

[0080] Examples of viscosity modifiers are polybasic carboxylic acidsand their esters or polyhydric alcohols.

[0081] The fibers of the present invention, especially themonofilaments, are preferably used for producing textile fabrics, suchas wovens, formed-loop knits, drawn-loop knits, non crimp fabrics andnonwovens.

[0082] Textile fabrics containing monofilaments of the present inventionare especially useful for industrial applications, as for filters, asscreen printing materials or especially as paper machine wires.

[0083] The monofilaments of the present invention have goodtextile-physical properties and are easy to process by weaving. Thewovens have the usual shape stability of the thermoplastic polymerswhich form the core.

[0084] Wovens formed from these monofilaments are very useful forindustrial cloths, especially in the filtration of aggressive mediawhere there is also a risk of an electrostatic charge buildup; that is,especially in solid-gaseous and solid-liquid filtration.

[0085] The invention also includes the use of the fibers for producingtextile fabrics which are used in environments featuring severe chemicaland/or physical stress, especially as paper machine wires or industrialwovens, as for example in filtration, for producing conveyor belts or asreinforcing plies. Here the fibers are used as monofilaments andespecially as weft threads in the woven fabric.

[0086] The use of the monofilaments of the present invention as papermachine wires can take place in the forming section, the pressingsection or in particular the drying section. When used in the dryingsection, the monofilaments of the present invention are used as spiralwires in particular.

[0087] For these applications, the fibers used according to the presentinvention, especially in the form of monofilaments, typically have alinear density range from 10 to 4,500 tex, an elasticity modulus of 2.0to 8.0 N/tex, a tenacity of 15 to 50 cN/tex, a breaking extension of 15to 45% and a 180° C. hot air shrinkage of 1.0 to 20.0%.

[0088] The example which follows illustrates the invention withoutlimiting it.

[0089] Core-sheath fibers composed of polyethylene terephthalate andpolyvinylidene fluoride containing carbon black.

[0090] Polyethylene terephthalate (PET) (70% by weight) andpolyvinylidene fluoride (as a masterbatch containing 9% by weight ofconductivity carbon black; Palmarole EXP 184/14; 30% by weight) weremelted in two extruders featuring separate temperature control (PET at282° C. melting temperature (core material) and PVDF at 240° C. meltingtemperature (sheath material) and spun through a 20 hole spinnerethaving a hole diameter of 1.40 mm and a hauloff speed of 15 m/min toform a core-sheath monofilament, doubly drawn (first drawing in waterbath at 80° C.; second drawing in hot air duct at 150° C.) and also heatset in the hot air duct at 205° C. The overall draw ratio was 4.1:1. Thefinal diameter of the core-sheath monofilament was 0.500 mm.

[0091] The core-sheath monofilament obtained had the followingproperties: Linear density: 2890 dtex Tenacity:   24 cN/tex 160° C. hotair shrinkage 10′: 2.5% Loop tenacity: >20 cN/tex Breaking extension: 44% 12 cN/tex EASL: 8.5% 15 cN/tex EASL:  13% El. resistance (10 mmclamped length): 8 * 10⁵ (ohm) El. resistance (150 mm clamped length):9 * 10⁶ (ohm)

What is claimed is:
 1. A melt-spun fiber having a core-sheath structureand a tensile strength of at least 15 cN/tex whose core contains asynthetic thermoplastic polymer which is not a fluoropolymer and whosesheath contains at least one melt-spinnable fluoropolymer and particlescomprising electroconductive material.
 2. The melt-spun fiber of claim1, wherein the synthetic thermoplastic polymer of the core is apolyamide and especially a polyester.
 3. The melt-spun fiber of claim 2,wherein the polyester is a polyethylene terephthalate.
 4. The melt-spunfiber of claim 2, wherein the polyester is a liquid-crystallinepolyester.
 5. The melt-spun fiber of claim 1, wherein the melt-spinnablefluoropolymer is a copolymer of tetrafluoroethylene with at least onealpha-olefin, preferably ethylene.
 6. The melt-spun fiber of claim 1,wherein the melt-spinnable fluoropolymer is a polyvinylidene fluoride.7. The melt-spun fiber of claim 1, wherein the sheath contains up to 50%by weight and preferably 2 to 15% by weight of electroconductiveparticles.
 8. The melt-spun fiber of claim 6, wherein the sheathcontains between 2% by weight and 15% by weight and especially between4% by weight and 9% by weight of electroconductive particles.
 9. Themelt-spun fiber of claim 1, wherein the particles comprisingelectroconductive material consist of carbon, metals or metal alloys andare especially carbon black or graphite.
 10. The melt-spun fiber ofclaim 1 which is a filament, especially a monofilament.
 11. A processfor preparing the melt-spun core-sheath fiber of claim 1, comprising themeasures of: i) selecting a first polymer which is a syntheticthermoplastic polymer and not a fluoropolymer and which has a firstmelting point, ii) selecting a second polymer which contains particlescomprising electroconductive material and which is a melt-spinnablefluoropolymer which has a second melting point at least 20° C. below thefirst melting point, iii) coextruding the first polymer and the secondpolymer through a heterofilament spinneret at a spinning temperatureabove the first melting point to form a bicomponent fiber having a corecomprising the first polymer and a sheath comprising the second polymerand iv) drawing the produced core-sheath filament to increase thetensile strength.
 12. The use of melt-spun core-sheath fibers of claim 1for producing industrial wovens.
 13. The use of claim 12, wherein theindustrial woven is a paper machine wire, a filter cloth, a screenprinting cloth, a conveyor belt or a reinforcing ply.